The full-length cDNAs for human LDH-B (heart), mouse LDH-B and LDH-C (testis) isoenzymes have been cloned and their nucleotide sequences determined. Human genomic clones containing LDH-B gene have been isolated and the exon-intron organization of the gene is being characterized. Human LDH-C gene has been mapped on chromosome no. 11 and human genomic clones containing LDH-C gene-related sequences are being isolated and characterized. The complete structure of mouse LDH-A gene has been determined, and its promotor region fused with gpt gene was shown to be functional in CHO cells. Further, the expression of mouse LDH-A promoter fused with cat gene was also shown to be induced by cyclic AMP and estrogen, and their regulatory elements were tentatively identified by sequence comparison with those of other mammalian genes. The and molecular nature of spontaneous mutations present in these pseudogenes were analyzed. The information obtained from these studies will allow more accurate evaluation of genetic mutation events caused by environmental agents and eventually will be of value to improve human health care.